Long range radio communication equipment often operates at frequencies within the L.F. (low frequency) and V.L.F. (very low frequency) bands, hereinafter referred to collectively as the "low frequency bands." Submarine radio systems, for example, usually utilize carrier frequencies within the range of approximately three kilohertz to three hundred kilohertz since the underwater propagation of radio signals is particularly practical at these frequencies. The utilization of a frequency of twenty kilohertz is common within this band.
Fixed or rotatable loop antennas are generally employed with such low frequency radio communication systems. Antennas of this type have been found to have relatively good performance characteristics in a water environment with respect to parameters such as signal-to-noise ratio, directivity paterns, and signal strength.
Tuning of the antenna for operation at a specific frequency within the band is customarily accomplished by modifying the reactance of a variable capacitive impedance element. This variable capacitive reactance element is normally connected across the loop antenna to allow the selective tuning of the antenna for operation at a particular frequency with the band. Because wavelength is inversely proportional to frequency, both the loop antenna and the variable capacitive reactance element in communication systems operating within the low frequency bands require relatively large inductive and capacitive values. Large physical dimensions are, of course, generally required to obtain these large inductive and capacitive values.
In an application Ser. No, 556,790, filed June 10, 1966, now U.S. Pat. No. 3,528,014, the low frequency communication systems then known, and particularly the underwater V.L.F. radio receiving systems then known, were modified by supplementing the variable tuning capacitive reactance with a variable negative impedance. While an output signal amplitude greater than that of similar antenna systems then known resulted, the increase in signal strength was accompanied by a smaller operating bandwidth.
It is accordingly an object of the present invention to provide a novel frequency independent antenna.
It is another object of the present invention to provide a novel method and apparatus for increasing the effective bandwidth of a loop antenna.
Yet another object of the present invention is to effect the aforementioned increase in effective operating bandwidth by the insertion in such antenna systems of circuit means having relatively small physical size and weight.
A further object of the present invention is to provide a novel method and apparatus for increasing the terminal signal voltage of a loop antenna system operating in the low frequency bands while maintaining broadband selectivity.
A still further object of the present invention is to provide a novel method and apparatus for obviating the necessity for tuning an antenna with a capacitor.
Yet a further object of the present invention is to provide a novel method for tuning a loop antenna to a range of frequencies extending over several octaves.
Briefly, the present invention contemplates accomplishing the foregoing objects by coupling a negative inductive reactance circuit means with a loop antenna system to effectively cancel the effects of frequency on the inductive reactance of the loop antenna. In the embodiment described, the antenna is a plural turn inductor which is capable of radiating electromagnetic energy in the low frequency bands. By providing a negative inductive reactive circuit in conjunction with an antenna which inherently has inductance, the antenna becomes broadband and the necessity of tuning the antenna to an individual frequency is obviated. The resultant signal provided by the antenna system of the present invention is fed to the active input stage of the receiver for processing in a conventional manner.
These and many other objects and advantages will be readily apparent to one skilled in the art to which the invention pertains from the following description of a preferred embodiment when read in conjunction with the accompanying drawings.